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Axiom ax-11o 1701
 Description: Axiom ax-11o 1701 ("o" for "old") was the original version of ax-11 1394, before it was discovered (in Jan. 2007) that the shorter ax-11 1394 could replace it. It appears as Axiom scheme C15' in [Megill] p. 448 (p. 16 of the preprint). It is based on Lemma 16 of [Tarski] p. 70 and Axiom C8 of [Monk2] p. 105, from which it can be proved by cases. To understand this theorem more easily, think of "¬ ∀xx = y →..." as informally meaning "if x and y are distinct variables then..." The antecedent becomes false if the same variable is substituted for x and y, ensuring the theorem is sound whenever this is the case. In some later theorems, we call an antecedent of the form ¬ ∀xx = y a "distinctor." This axiom is redundant, as shown by theorem ax11o 1700. This axiom is obsolete and should no longer be used. It is proved above as theorem ax11o 1700. (Contributed by NM, 5-Aug-1993.) (New usage is discouraged.)
Assertion
Ref Expression
ax-11o x x = y → (x = y → (φx(x = yφ))))

Detailed syntax breakdown of Axiom ax-11o
StepHypRef Expression
1 vx . . . . 5 setvar x
2 vy . . . . 5 setvar y
31, 2weq 1389 . . . 4 wff x = y
43, 1wal 1240 . . 3 wff x x = y
54wn 3 . 2 wff ¬ x x = y
6 wph . . . 4 wff φ
73, 6wi 4 . . . . 5 wff (x = yφ)
87, 1wal 1240 . . . 4 wff x(x = yφ)
96, 8wi 4 . . 3 wff (φx(x = yφ))
103, 9wi 4 . 2 wff (x = y → (φx(x = yφ)))
115, 10wi 4 1 wff x x = y → (x = y → (φx(x = yφ))))
 Colors of variables: wff set class This axiom is referenced by: (None)
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